1. Field of Invention
This invention pertains to the art of methods and apparatuses for forming gears, and more specifically to methods and apparatuses for forming a sliding gear with powder metallurgy (PM).
2. Description of the Related Art
Powder metallurgy (PM) is a known method for fashioning metal parts. The PM process utilizes powdered metal that is added to a mold and then compacted under very high pressures, usually above 20 tons per square inch. The resultant part comes out of the mold as a xe2x80x9cgreenxe2x80x9d part. The PM process is completed by sintering the xe2x80x9cgreenxe2x80x9d part in a furnace, usually at temperatures between 2000xc2x0 and 2500xc2x0 F. The sintering process essentially welds together the compacted powdered metal of the green part into a solid mass.
Powder metallurgy (PM) has proven to provide advantages over prior methods of forming metal parts especially in the area of cost efficiency. There is a problem, however, related to the requirement for tool shut off shelves (also known as a PM chamfers). These shelves are known to be required because the tools used in forming the part must be terminated perpendicular to the direction of tool movement. The problem in the use of PM to form gears is that the tool shut off shelves interfere with the intermeshing of the gear teeth.
To illustrate this problem, FIG. 5 shows a top view of a pair of sliding gear teeth 10, 10 made in accordance with a known PM process. Each tooth includes sliding surfaces 20 that are intended to slide against one another as the teeth slidingly intermesh. Each tooth also has tool shut off shelves 14 as known PM processes require. The problem is that the shut off shelves 14 form protrusions 26 that prevent the proper intermeshing of gear teeth 10 because the protrusions 26 of one tooth physically interfere with the opposite tooth as one gear slides toward the other gear. One possible way to alleviate this problem is to remove the protrusions 26 such as by machining them off the teeth 10. This is an expensive and undesirable extra operation in the manufacturing process. For this reason the PM process is not known to be used in forming gears having slidingly intermeshing gear teeth.
The present invention provides methods and apparatuses for forming sliding gears using the PM process without any such extra step. The difficulties inherent in the related art are therefore overcome in a way that is simple and efficient, while providing better and more advantageous overall results.
According to one aspect of this invention, a gear tooth is provided that has a main body and a lead-in portion. The lead-in portion is used to receive a lead-in portion of a corresponding gear tooth on another gear. The main body includes first and second ends and first and second sides. The sides form first and second protrusions, respectively, with the first end. The lead-in portion includes first and second side walls extending from the first end of the main body and first and second sliding surfaces extending from the first and second side walls respectively. The first sliding surface lies on a first plane. The first side wall defines a first shelf on the first end of the main body. What is especially to be noted is that the first protrusion, the first shelf and the first side of the main body are all positioned between the first plane and the main body""s axial centerline CL.
According to anther aspect of this invention, a method of forming a gear is provided. First, powdered metal is added to a gear mold. The powdered metal is then compacted within the gear mold. Next, a green gear is formed. The green gear has one tool shelf around the perimeter of the gear. Thus each of the gear teeth has a tool shut off shelf. Each of these tool shut off shelves are adapted to not interfere with a similarly formed gear in sliding engagement. Finally, the green gear is sintered (heated) so that the powdered metal of the green part fully solidifies into a single mass. It should be noted that this method removes the currently required step of removing the tool shut off shelves on the gear teeth.
According to still another aspect of this invention, the tool shut off shelves and side walls can be reduced in size to the point where they are substantially eliminated. Thus, no interference problems are encountered during sliding intermeshing with another similarly formed gear.
One advantage of the current invention is that the gear teeth easily slide over each other and thereby properly mesh.
Another advantage of the current invention is that the current invention allows these desirable gear teeth to be produced by the inexpensive and cost efficient process of powder metallurgy.
Another advantage of the current invention is that the undesirable step of machining or otherwise reducing the protrusions formed on gear teeth in the powdered metallurgy process is removed.
Still other benefits and advantages of the invention will become apparent to those skilled in the art to which it pertains upon a reading and understanding of the following detailed specification.